The invention relates to a positive radiation-sensitive mixture containing a compound which forms an acid under the action of high-energy radiation, and an acid-cleavable compound.
In classical UV lithography, the resolution limit is determined by the wavelength of the radiation used. The constant decrease in dimensions in chip production therefore requires new lithographic techniques in the submicron region, electron or X-ray radiation being employed due to its extremely short wavelength. At the same time, it has been shown that resist materials which are suitable as electron beam resist can also be employed as X-ray resist, and vice versa.
Known resist materials for this application are acrylates and methacrylates (G.M. Taylor, Solid State Technology, 124 (1984)). In the case of these materials, it has been shown that sensitivity and structural resolution are usually contrary properties. If it is to be possible for higher sensitivities to be achieved, halogens are usually incorporated into the resist. In this case, fluorine and chlorine are usually employed in positive resists, whereas it is usually bromine and iodine, besides chlorine, which are employed in negative resists (T. Yamaoka et al., Phot. Sci. Eng. 23. 196 (1979)).
In general, negative resists exhibit higher sensitivity than positive resists, but, in contrast, cannot simultaneously--as stated above--have a high resolution in the submicron region. On the other hand, positive methacrylate-based resists achieve high resolution, but, with the exception of polymethacrylonitrile-based resists, are not stable to the plasma etching processes used for semiconductor structuring. In turn, however, the methacrylates are not sufficiently sensitive.
The polymers having the highest radiation sensitivity known to date to electron beams or X-rays are polyalkene sulfones, in particular polybutene 1-sulfone. The disadvantage of this class of compounds is, however, that they are less resistant to plasma etching processes; they are therefore suitable for mask production, but not for semiconductor fabrication using a mask made of this material. It has therefore been proposed to combine polyalkene sulfones with novolak resins, which, as is generally known, are resistant to plasma etching (M. J. Bowden et al., J. Electrochem. Soc. 128, 1304 (1981); U.S. Pat. No. 4,289,845). However, it became apparent that the two polymers are extremely incompatible with one another, thus impairing the resolution. An attempt to improve the compatibility by admixing further components also had to pay the price of loss in sensitivity (U.S. Pat. No. 4,398,001).
Photocatalytic systems are described in DE-A-2,718,254 (=U.S. Pat. No. 4,247,611) and DE-A-2,928,636 (=U.S. Pat. No. 4,311,782) for use with electron and X-ray radiation. In these positive systems, chlorinecontaining compounds, in particular of the substituted triazine type, are employed as compounds which form an acid under the action of actinic radiation. During structuring of these materials using electron or X-ray radiation, however, it was apparent that the edges of the resist structures are very negatively eroded after development (edge angle around 60.degree. according to a photomicrograph using a scanning electron microscope (SEM)) and structures of less than about 2 .mu.m consequently can no longer be resolved or imaged or, in some cases, are extremely frayed.
In DE-A-2,928,636, as in DE-A-2,610,842 (=U.S. Pat. No. 4,101,323), 2,3,4,5-tetrachloroanaline, inter alia, is mentioned as the chlorine-containing compound. In addition, DE-A-2,610,842 also discloses compounds which contain aliphatically bound bromine and one which carries aromatically bound bromine: 2,2',4,4',6,6'-hexabromodiphenylamine.
Chlorine- and bromine-containing compounds in photo-sensitive materials also include hexabromodimethyl sulfoxide in photographic materials (DE-B 1,572,089=GB-A 1,163,324), 2,2-dibromo-2-(phenylsulfonyl)acetophenone and related compounds as polymerization initiators (DE-A 1,817,540=U.S. Pat. No. 3,615,455), trichloromethyl-substituted benzophenones as polymerization initiators (DE-A-1,949,010 =U.S. Pat. No. 3,686,084), haloalkylbenzoxazoles, -benzimidazoles and -benzothiazoles as polymerization initiators (U.S. Pat. No. 3,912,606) and side group-halogenated acetophenones as photocatalysts for acid-curable surface coatings (DE-A 2,317,846).
A number of these compounds result in significant losses in resolution. Without exception, undesired image fogging occurs, which excludes use of such compounds in practice as photolytic acid donors in X-ray lithography. The chlorine compounds in particular exhibit inadequate resolution. By contrast, the compounds mentioned containing aromatically bound bromine cause undesired image fogging. In general, however, this image fogging is attributable to inadequate solubility of the initiator compounds in aqueous alkaline developers.
It has therefore been proposed, in accordance with the earlier German Patent Application P 38 21 585.3, to use as initiator compounds, compounds containing aromatically bound chlorine or bromine which have a pK.sub.a value of less than 12. Although the compounds mentioned exhibit good sensitivities on irradiation with synchrotron radiation, the sensitivities which can be achieved using them on irradiation with UV radiation are, however, relatively low.